(1) Field of the Invention
This invention relates to a light receiving device, and more particularly to a light receiving device suitable for use with a receiving system of an optical communication system. More specifically, the present invention relates to a light receiving device suitable for use with a receiving system of an optical communication system which performs continuous transmission.
(2) Description of the Related Art
In a popular optical communication system, a light signal transmitted through an optical fiber which forms a transmission line is received, for example, by such a light receiving device 100 as shown in FIG. 11. In the light receiving device 100, the light signal transmitted through the transmission line is received and converted into a current signal by a photoelectric conversion element 101 and is then converted into and amplified as a voltage signal by a pre-amplifier 102. Then, the voltage signal is further amplified by a main amplifier 103 and outputted to a clock and data recover (CDR) circuit 104 for reproducing a clock signal and a data signal.
At this time, the main amplifier 103 converts the voltage signal from the pre-amplifier 102 with a predetermined decision level A into another voltage signal of a rectangular waveform having voltage levels representative of a magnitude with respect to the decision level A and outputs the voltage signal of the rectangular waveform to the CDR 104. In other words, the main amplifier 103 can detect modulation data as a voltage signal of a substantially rectangular waveform.
In order to achieve continuous transmission wherein a light signal modulated with data is continuously inputted, the light receiving device 100 is formed using an AC (Alternating Current) coupling. More particularly, a capacitance element for removing direct current is provided inside the pre-amplifier 102 or between the pre-amplifier 102 and the main amplifier 103 so that a DC component included in a reception signal can be removed. The CDR 104 performs reproduction of a data signal and a clock signal based on the signal waveform from which a DC component has been removed.
It is to be noted that, while the electric signal obtained by the conversion by the photoelectric conversion element 101 exhibits, after a DC component is removed therefrom, an AC signal waveform having a 0 level substantially in the proximity of the center of the signal amplitude as seen in FIG. 12, but exhibits, before a DC component is removed therefrom, a signal waveform which has a 0 level substantially in the proximity of the bottom of the signal amplitude and has levels corresponding to emission/no-emission of the light as seen in FIG. 13.
On the other hand, in order to perform burst transmission wherein a light signal modulated with data is inputted intermittently, since it is demanded to receive signals of various high and low levels and the mark ratio does not become equal to 1/2, the light receiving device 100 is formed using a DC (Direct Current) coupling. In particular, in the light receiving device 100 described above, a capacitance element for removing direct current is not provided inside the pre-amplifier 102 or between the pre-amplifier 102 and the main amplifier 103, but the electric signal obtained by photoelectric conversion of the light signal by the photoelectric conversion element 101 is used to reproduce a data signal and a clock signal based on such a signal waveform having levels corresponding to on/off of light as described hereinabove with reference to FIG. 13.
Incidentally, an index which is significant as a characteristic of a light receiving device is a reception sensitivity characteristic. In particular, the significant index is a minimum reception sensitivity on the minimum reception side and an OSNR (Optical Signal to Noise Ratio) tolerance on the maximum reception side. The minimum reception sensitivity is a minimum reception power with which the BER (Bit Error Rate) satisfies a required specification when a signal on the side where the level is comparatively low is received where the OSNR is fixed, for example, as illustrated in FIG. 14. Meanwhile, the OSNR tolerance is a minimum OSNR with which the BER (Bit Error Rate) satisfies a required specification when a signal on the side wherein the level is comparatively high is received.
It is to be noted that a straight line F of the BER with respect to the reception power illustrated in FIG. 14 and a straight line G of the BER with respect to the OSNR illustrated in FIG. 15 are called error straight lines.
The burst transmission which uses a DC coupling is a transmission system used in an access type optical network and does not require taking the OSNR tolerance into consideration because the transmission distance is short and hence the OSNR little suffers from degradation. However, since ordinary continuous transmission which uses an AC coupling is used for long distance transmission by a trunk transmission network or a metro network, degradation of the OSNR is a significant problem and optimum decision level control with the OSNR taken into consideration is essentially required. It is known that optical noise by degradation of the OSNR usually appears much on the mark side of a signal.
Incidentally, while various techniques relating to a reception system of an optical communication system are known, those techniques disclosed in Japanese Patent Laid-Open No. Sho 60-197051 (hereinafter referred to as Patent Document 1), No. 2003-018140 (hereinafter referred to as Patent Document 2) and No. Hei 9-270755 (hereinafter referred to as Patent Document 3) are known as techniques which are related to the invention of the present application.
The techniques disclosed in Patent Document 1 and Patent Document 2 presuppose detection of the BER and feedback control of the decision level. Meanwhile, according to the technique disclosed in Patent Document 3, where the amplitude of data received by a light receiver having a configuration of a DC coupling is within a nonlinear range of a pre-amplifier, the threshold voltage for a main amplifier is changed over to a value higher than the center of the amplitude of the received data to correct the duty ratio.
However, with such techniques disclosed in Patent Document 1 and Patent Document 2 as described above, since a large scale circuit is required in order to detect the BER, there is a subject to be solved in that it is actually difficult to incorporate the apparatus in a light receiving device.
Meanwhile, with the light receiver disclosed in Patent Document 3, since it does not have a circuit configuration of an AC coupling, it does not have a configuration for improving the reception characteristic of a light receiving device when continuous transmission is performed.